The present invention relates to the general field of turbine engines and it applies in preferred manner to the field of aviation.
The invention relates more particularly to regulating the flow rate of fuel to an aircraft turbine engine, such as a turbojet for example, during a transient stage of operation for the aircraft, such as a stage of acceleration or of deceleration.
In known manner, the fuel flow rate for a turbojet is regulated for the purpose of ensuring that the mass flow rate of fuel injected into the combustion chamber of the turbojet during a stage of acceleration or of deceleration does not cross a certain limit value beyond which it is possible that the turbojet will malfunction. By way of example, such malfunctioning may be surging of a compressor of the turbojet during a stage of acceleration or burning out of the turbojet during a stage of deceleration.
Nowadays, this regulation relies on an estimate of the mass flow rate of fuel injected into the combustion chamber as established from the position of the fuel metering device of the turbojet.
More precisely, the fuel metering device has a slide of position, also called fuel metering valve (FMV), whose position is proportional to the volume flow rate of fuel that is to be injected into the combustion chamber, providing the pressure difference across the slide is kept constant.
On moving, the slide obstructs a fuel flow section S in the fuel metering device to a greater or lesser extent. The section S, also referred to as the flow area of the fuel metering device, can easily be determined as a function of the position of the slide. It is proportional to the volume flow rate of fuel injected into the combustion chamber.
The section S is controlled via a servo-control loop by means of an electronic engine control unit (ECU) forming part of the full authority digital engine control (FADEC) of the aircraft. The ECU establishes the mass flow rate demand for fuel in the form of a setpoint signal that is transmitted to the fuel metering device via the servo-control loop, this mass flow rate demand for fuel then being transformed into a volume flow rate.
The mass flow rate of fuel injected into the combustion chamber can thus be estimated on the basis of measuring the position of the fuel metering device and of an associated relationship that is known to the person skilled in the art. This relationship depends on the density (i.e. the mass per unit volume) of the fuel used by the aircraft.
The density of the fuel is generally assumed to be constant during a mission of the aircraft. It is predetermined as a function of reference conditions, i.e. for a reference fuel and at a reference temperature.
Nevertheless, this assumption does not take account of a possible change in the nature of the fuel used relative to the reference fuel, nor does it take account of possible variation in the temperature of the fuel relative to the reference temperature while the turbojet is in operation.
Nor does it take account of the fact that, for a given fuel, there may exist dispersion in density value around the reference value used in the relationship.
Furthermore, this assumption suffers from various uncertainties associated in particular with the interchangeability of the pieces of electronic equipment used for processing position measurements of the metering device and also associated with the accuracy of those pieces of equipment.
Consequently, using such a relationship leads to inaccuracies in the estimated mass flow rate of fuel injected into the combustion chamber as used while regulating the turbojet.
Unfortunately, if the mass flow rate of fuel being injected into the combustion chamber is underestimated by the fuel metering device, that means that the acceleration margin available to the turbojet during a transient stage of acceleration will on the contrary be overestimated (i.e. the turbojet will be presumed to have an acceleration margin greater than that which would be presumed if the fuel mass flow rate were estimated correctly). There is thus a risk of undesired acceleration of the turbojet, and potentially of the compressor surging during acceleration.
In an opposite manner, the deceleration margin of the turbojet during a deceleration stage will be underestimated (i.e. the turbojet will be presumed to have a deceleration margin that is smaller than that which would be presumed if the fuel mass flow rate were estimated correctly). There is thus a risk of not being able to decelerate correctly.
Conversely, if the mass flow rate of fuel being injected into the combustion chamber is overestimated by the fuel metering device, that means that the deceleration margin available to the turbojet during a transient stage of deceleration will be overestimated (i.e. the turbojet will be presumed to have a deceleration margin that is greater than that which would be presumed if the fuel mass flow rate were estimated correctly). There thus exists a risk of the turbojet suffering underspeed or burnout.
In an opposite manner, the acceleration margin of the turbojet in an acceleration stage will be underestimated (i.e. the turbojet will be presumed to have an acceleration margin that is smaller than that which would be presumed if the fuel mass flow rate were estimated correctly). There thus exists the risk of not being able to accelerate correctly.
The lack of accuracy in the estimate provided by the metering device thus has a non-negligible impact on the regulation of the turbojet and on the performance it achieves.
There therefore exists a need to regulate the turbojet in a manner that does not present such drawbacks.